That low hum at night, a whine from the walls, a buzz nobody else hears. Record it with your microphone and this finds the dominant pitches and matches them to likely sources: mains electricity (50/60 Hz and harmonics), transformers, fridge compressors, fans, coil whine, ultrasonic pest repellers and more — giving you something concrete to hunt for or report.
Get close to where the sound seems loudest; keep still and quiet while it listens.
Audio is analysed live in your browser and never recorded to a file or uploaded. Phone and laptop microphones roll off below ~40 Hz and above ~18 kHz, so very deep rumbles and true ultrasound may be invisible — absence of a peak isn’t proof of silence. A persistent low hum heard only by you can also be tinnitus; if no source is ever found, a hearing check is worthwhile.